In a large bowl, combine yogurt, balsamic vinegar, tahini, Tandoori Seasoning, honey, water, and salt. Remove 1 cup of sauce. Cover and chill until ready to use. Add chicken thighs to large bowl and coat in remaining sauce. Cover and chill for 1 hour. Preheat oven to 400 degrees with a single oven rack at the lowest level. Line a baking sheet with aluminum foil. Slice onion in half, peel away papery layers, and place one half, cut-side down, in the center of the baking sheet. Dice the remaining half and set aside. Stab wooden skewer all the way down into the center of the onion on the baking sheet so that it is sturdy and stands straight up. When finished marinating, remove thighs and stab centers through top of skewer. Stack thighs over the onion base, rotating to ensure even stacking. Top stack with the red bell pepper and break off any exposed portion of wooden skewer. Transfer baking sheet to oven and roast for 1 to 1 1/2 hours, or until center of chicken reaches 160 degrees and edges are charred. Remove from oven and rest for 10 min. Final temperature should reach 165 degrees. While chicken is resting, add roasted red bell pepper and red onion to a blender along with 1 cup of reserved sauce. Blend for 30 seconds, or until smooth, scraping sides as needed. Slice bite-size pieces of chicken off the skewer and add to medium bowl. Toss chicken in 1 cup of blended sauce. Fill warmed pitas with chicken, tomato, cucumber, and reserved diced onion and serve with remaining sauce.